1. Field of the invention
The present invention relates to a piezoelectric transducer suitable for use in an ultrasonic motor. Related art
The composition and operation of a related-art piezoelectric transducer used in an ultrasonic motor are explained hereinafter with specific reference to FIGS. 5 and FIG. 7.
A first elastic member 302a comprises a cylindrical portion 303a and a resonance disk 304a coaxially mounted on the cylindrical portion 303a. The first elastic member 302a is provided with a penetrating hole along an axis of symmetry of a piezoelectric transducer 301. A first piezoelectric element 306a is provided on the under surface of the cylindrical portion 303a. A third piezoelectric element 310a is provided on the under surface of the resonance disk 304a. A fixing plate 315 is installed under the first piezoelectric element 306a. A second elastic member 302b, a second piezoelectric element 306b, and a fourth piezoelectric element 310b are arranged below the fixing plate 315 so that the piezoelectric transducer 301 is symmetrical about the fixing plate 315.
The piezoelectric transducer 301 constructed as above is fastened together by a bolt 313 penetrating through the penetrating hole and a nut 314. The dimensions of the first and the second elastic members 302a and 302b are determined so that the first and second elastic members 302a and 302b resonate in a shear vibration mode (referred to as peripheral shear vibration hereinafter) and in a flexural vibration mode (referred to as axial flexural vibration hereinafter) at predetermined frequencies.
As shown in FIG. 6B, the first and second piezoelectric elements 306a and 306b are each composed of eight piezoelectric chips 201 arranged in a circle. As shown in FIG. 6B, each piezoelectric chip 201 is cut out of a piezoelectric board 200 in a shape forming one eighth of the circle. The piezoelectric board 200 for generating thickness shear mode vibration has a polarized direction indicated by arrow A as shown in FIG. 6A.
As illustrated in FIG. 7, the piezoelectric chips 201 are assembled in such an arrangement that the first piezoelectric element 306a is polarized in a direction opposite to that of the second piezoelectric element 306b. This arrangement of the piezoelectric chips 201 is suitable for causing torsional vibration of an elastic member. The curved arrows in FIG. 5 indicate the polarized directions of the first and the second piezoelectric elements 306a and 306b.
The piezoelectric transducer 301 is fixed in a motor case 317 via the fixing plate 315. An output shaft 319 is rotatably supported in the motor case 317. One end of the output shaft 319 protrudes out of the motor case 317. A rotor 321 is attached to the other end of the output shaft 319 inside the motor case 310. A spring 323 is interposed between the motor case 317 and the rotor 321, and pushes the rotor 321 onto the resonance disk 304a of the piezoelectric transducer 301.
In the piezoelectric transducer 301 constructed as described above, aluminum disks having the outside diameter of 60 mm and the thickness of 3.5 mm are used as the resonance disks 304a and 304b of the elastic members 302a and 302b, respectively. The amplitude in the peripheral direction of the peripheral shear vibration occurring with the resonance disks 304a and 304b is 0.1 .mu.m on the peripherals of the resonance disks 304a and 304b when 25 volts are applied to the first and second piezoelectric elements 306a and 306b. The resulting rotational speed of the output shaft 319 is as low as 10 r.p.m. Therefore, the piezoelectric transducer 301 having the first and second piezoelectric elements 306a and 306b arranged as shown in FIG. 7 is insufficient for a use in an ultrasonic motor due to its limited peripheral shear vibration amplitude.
Another problem of the related-art piezoelectric transducer is that its insulating material causes considerable viscous loss and, therefore, does not effectively transmit vibration when used as an insulator provided between piezoelectric elements.